Recently, separation of mixed gases using membranes, i.e., membrane separation of gases has attracted much attention in many respects such as effects of saving energy and simplification of apparatus and process for the separation, and utilization thereof has been considered in a variety of fields of technology including production of oxygen- or nitrogen-rich air from the air, separation and recovery of CO.sub.2, recovery of CO and H.sub.2, removal of NO.sub.2 and SO.sub.2 from combustion waste gas, purification or adjustment of H.sub.2 /CO known as synthesis gas in the so-called C.sub.1 -chemistry, separation and recovery of inert gases such as He from natural gas, and the like. The important features of the membrane which are decisive as to whether or not it can be practically used or whether or not it gains wide acceptance in these fields from economical viewpoint, for example, to include high gas separating ability, high gas flux, that is, high gas permeation rate of the permeate gas per unit surface area of the membrane, etc., and it is highly desired to develop membranes which are excellent in these characteristics.
This invention is contemplated to cope with the above-described requirements and provides a method for producing gas separation membranes which method is capable of having a high gas separating ability and a high gas flux as well as excellent mechanical properties.